Using diet soda in brownies as a substitute for eggs and oil reduces calories and fat content, making that option appealing for dieters. Vegetarians and vegans who don't eat eggs, as well as people with egg allergies can eat brownies made using soda as the liquid instead of eggs and oil, since soda pop contains no animal products. When running low on eggs, oil or milk, soda pop substitutes in a pinch. Many cooks suggest using soda pop as a liquid substitute in brownies or cake for convenience since it's easier to open a can than to crack eggs and measure multiple liquid ingredients.

To determine the right amount of soda pop to substitute for the liquid ingredients, measure the total volume of liquid ingredients that the box directions call for. Convert each egg to 1/4 cup of liquid. After calculating the total amount of liquid, measure that amount of soda pop and add it to the mix. Decrease the amount of baking time, testing for doneness starting five minutes before the directed baking time.

Use soda pop in homemade brownies as well. First mix together the dry ingredients. Avoid using soda in brownies made with melted baker's chocolate instead of cocoa powder because adding soda in place of the oil and eggs may interfere with the consistency of the melted chocolate, which needs some fat for smoothness. Add soda pop to the dry ingredients a little at a time until the batter reaches the right consistency. Bake in a greased pan, testing for doneness after about 30 minutes.

Although most soda pop brownie recipes call for cola-flavored soda, use any flavor on hand. Try cherry cola to add a slight hint of cherries to the batch, or lemon lime soda for a lighter flavor. Avoid using soda flavors you wouldn't drink while eating a chocolate brownie since the hint of flavor it adds may clash with that of the brownie. Open the soda pop before mixing the ingredients to release some of the carbonation. Pour the soda into the dry ingredients slowly and pause to allow foam to dissipate. Expect some small lumps in the finished batter. Mixing until the batter is completely smooth incorporates too much air and causes the brownies to peak in the middle.

Expect brownies made using soda pop instead of oil and eggs to crumble more and have a less dense texture. Because eggs act as a binder in baked goods, without them the dry ingredients won't hold together as well, producing a crumbly and spongy texture. Brownies made with soda pop rise higher from the release of carbon dioxide from the soda's carbonation and have less of the denseness of a traditional brownie. The texture resembles cake.